Following another video where I showed that you can make a handy emergency backup light from salvaged lithium cells, I got a few messages implying that with a load connected to a cell the charging may not stop and risk overcharging the cell.

To be fair it does look like that is happening, but in reality its down to the way the TP4056 and its many clones and variants indicate the end of charge state.

I set up an experimental rig to monitor current and voltage, and confirmed that the reason the charge never seems to end is because of the programmed current threshold the TP4056 chips use to detect end of charge.

When you set the desired charge current on these chips with the programming resistor it doesn't just set the charge current, but also a threshold of one tenth of that current that is used for both initial trickle charging to get an over-discharged call gently back up to 3V, and also the point at the end of charge when the current gradually drops to that lower current level.
If the load is above that level then the current will gradually tail off at the end of charge, but because it ends up powering the load directly it never reaches the end of charge threshold.

It will only be supplying enough current to drive the load and hold the cell at its nearly fully charged state, but will just not switch the LEDs to display charge completion.

With a load current lower than the end of charge threshold it will terminate the charge, but then kick back in again when the voltage drops to a slightly lower voltage, and top the cell up again. That results in the charge status LEDs slowly toggling back and forth.

The cell will not be charged above its voltage limit in either instance.


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